The polar bear and the brown bear are rather closely related, and they can still interbreed. In fact, the evolutionary split between the two species is estimated to have taken place less than half a million years ago. Their color has obviously adapted to the Arctic, but the real difference between polar bears and other bears is their diets. Polar bears eat blubber-laden seals and can store hundreds of pounds of fat in their bodies. Scientists have found changes in polar bear DNA that deal with the huge amount of cholesterol they carry.

One of the most obvious is a gene involved in cholesterol metabolism. Despite all the time since their split from a common ancestor, panda bears and brown bears have no differences in this gene. In contrast, polar bears have picked up nine different changes in this gene in the 400,000 years or so that they've been breeding separately.

Whatever it's doing, the modified protein isn't keeping the bear's cholesterol down; the authors note that "Cholesterol levels in blood plasma of polar bears are extreme." Instead, the bear's evolution seems to have reworked the heart to survive these extreme cholesterol levels. Nine of the 16 genes that are changing the most in response to selective pressure are involved in cardiovascular development or maintenance. A few of the rest are involved in forming adipose tissue—presumably to get the bear up to the 50 percent fat figure noted above.